It just goes to show that in many ways it's the angler not the gear that makes a given set-up work. Myself, for general open-water stillwater fishing ( roach, bream, perch, small to medium tench, rudd, crucians) |I can't imagine putting a 6lb reel line on a trad match rod. I'm leaving carp and power float rods out of the equation. 2.5 to 3lb reel lines, not pre-stretched, are more than adequate. I notice that I favour a 2bb+ float where many go for a 2AA+, and I definitely find a thin line casts a small float better, besides balancing rod, hooklength and hooks more seamlessly.
Some lines are easier to sink than others. I don't know what others find, but I find Supplex, a line I like on a pin/long rod for fishing close'ish in floats like a cork. Manageable with a couple of yards in the water, but hopeless several rod lengths out. Good with a stick float, though. Any line can be sunk more easily if it's de-greased to remove stuff picked up from manufacture or the water surface.. I think that has got a bit lost now that so many of us fish poles or feeders where we once would have fished waggler. A degreased line can be chopped under with much less fuss - striking with a sunk rod tip, winding with the tip sunk and all that - than one not.
Do thin lines sink more quickly that thicker ones? We might think so, but it's not uncommon for anglers to use a much thicker than normal hooklength for catching fish on the drop; the thicker line, it's claimed, sinks more slowly.
When you've got your line to sink, there's another thing to think about. How far do you want it to sink? Some claim that lines that are happy to sink, like Maxima, sink too deep and impede the strike fractionally. I use it and it's never bothered me, but then on shallowish stills I much prefer to sweep the rod low to one side rather than up. Pulling the line through the water moves the float more effectively than lifting the line up. Your float is never going to shoot out of the water, as can happen with an upward strike and a hallow rig, nor do fish get dragged splashing to the top.